JP H3-79582 B discloses a gear transmission apparatus (A) which can change a direction of an output shaft (15) with respect to an input shaft (1). A bevel gear (4) is coupled to the input shaft (1), and engages an idler bevel gear (7) on an intermediate shaft (8), and the idler bevel gear (7) further engages an output-side bevel gear (10) coupled to the output shaft (15). The input shaft (1) is supported on an input-side casing (6) via a bearing, and an intermediate shaft cover (20) is fixed on the input-side casing (6). In addition, the output shaft (15) is supported on an output-side casing (17) via a bearing, and an intermediate shaft cover (22) is fixed on the output-side casing (17). The two intermediate shaft covers (20, 22) have thrust rings (21, 23) on respective surfaces that oppose each other. The thrust ring (21, 23) can slide in a circumferential direction, and, with this motion, the two intermediate shaft covers (20, 22) are relatively rotatable around an axis of the intermediate shaft (8). With the rotation of the intermediate shaft covers (20, 22), the output shaft (15) can rotate with respect to the input shaft (1) in a plane orthogonal to the axis of the intermediate shaft (8). In the above description, reference numerals and signs in the parentheses are those used in JP H3-79582 A, and do not correspond to the reference numerals or signs used in the description of the embodiment of the present disclosure.
In the gear transmission apparatus described in JP H3-79582 A, the output shaft is rotatable only in one plane with respect to the input shaft, and a degree of freedom of rotation as a joint is 1. An advantage of the present disclosure lies in provision of a gear transmission apparatus having 2 degrees of freedom of rotation as a joint.